Ion conductive material for secondary battery

ABSTRACT

A secondary battery is comprised of a positive electrode having a material intercalating alkali earth metal ions or lanthanoid metal ions reversibly, a negative electrode having a carbon composition intercalating those ions reversibly and an electrolyte having an organic solvent and a solute. The solute includes an alkali earth metal salt or a lanthanoid metal salt. The organic solvent is selected from a group comprising ethylene carbonate (EC), dimethyl carbonate (DMC) and vinylene carbonate (VC). The carbon composition is selected from a group comprising coke, refined coke with 99% or more purity, organic compound produced by calcined cellulose, graphite and glassy carbon.

This is a divisional of application Ser. No. 08/466,197, filed Jun. 6,1995, now U.S. Pat. No. 5,601,949, which is a continuation-in-part ofapplication Ser. No. 08/134,079, filed Oct. 8, 1993, now abandoned.

BACKGROUND OF THE INVENTION

1. Field of the invention

This invention relates to a secondary battery, and more particularly toan improved ion conductive material for an active material to be used ina secondary battery.

2. Description of the Prior Art

Currently, a negative electrode having carbon compositions for asecondary battery using lithium provides certain advantages. Forexample, such secondary battery has a higher capacity than a Ni--Cdbattery. Although carbon compositions using lithium have superiordischarge capacities in comparison to Ni--Cd batteries, this advantageis limited due to the use of +1 lithium ions. Also, such secondarybatteries can discharge for a long period on one charging; therefore,they are energy-saving batteries.

However, in order to achieve superior characteristics, a secondarybattery having an even higher capacity is needed.

SUMMARY OF THE INVENTION

Accordingly, a primary object of the present invention is to provide asecondary battery with improved battery characteristics, such asimproved discharge capacity.

Another object of the present invention is to provide a secondarybattery with improved discharge voltage.

In accordance with one specific embodiment of the invention, the aboveobjects are fulfilled by a secondary battery comprising a positiveelectrode having a material intercalating alkali earth metal ionsreversibly, a negative electrode having a carbon compositionintercalating alkali earth metal ions reversibly and an electrolytehaving an organic solvent and a solute. The solute includes an alkaliearth metal salt, for example, Ba(BF₄)₂, Ba(CF₃ SO₃)₂, Ba(PF₆)₂,Ba(ClO₄)₂, Ba(AsF₆)₂, Ba(SbF₆)₂, Sr(BF₄)₂, Sr(CF₃ SO₃)₂, Sr(PF₆)₂,Sr(ClO₄)₂, Sr(AsF₆)₂, Sr(SbF₆)₂, Ca(BF₄)₂, Ca(CF₃ SO₃)₂, Ca(PF₆)₂,Ca(ClO₄)₂, Ca(AsF₆)₂, Ca(SbF₆)₂, Mg(BF₄)₂, Mg(CF₃ SO₃)₂, Mg(PF₆)₂,Mg(ClO₄)₂, Mg(AsF₆)₂ and Mg(SbF₆)₂. The material for the positiveelectrode is selected from BaNiO₃, BaNiO₂, BaCoO₃, BaCoO₂.8, BaFeO₃,SrNiO₃, SrCoO₂.5, SrCoO₂.8, SrCoO₃, SrFeO₄, SrFeO₂.5, SrFeO₃, CaCo₂ O₄,Ca₃ Co₄ O₉, Ca₂ Co₂ O₅, Ca₃ Co₂ O₆, CaFeO₃, CaFeO₂, MgNiO₂, MgCo₂ O₄ andMgFe₂ O₄.

In accordance with another specific embodiment of the invention, theabove objects are fulfilled by a secondary battery comprising a positiveelectrode having a material intercalating lanthanoid metal ionsreversibly, a negative electrode having a carbon compositionintercalating lanthanoid metal ions reversibly and an electrolyte havingan organic solvent and a solute. The solute includes a lanthanoid metalsalt, for example, Sm(BF₄)₃, Sm(CF₃ SO₃)₃, Sm(PF₆)₃, Sm(ClO₄)₃,Sm(AsF₆)₃, Sm(SbF₆)₃, Eu(BF₄)₃, Eu(CF₃ SO₃), Eu(PF₆)₃, Eu(ClO₄)₃,Eu(AsF₆)₃, Eu(SbF₆)₃, Yb(BF₄)₃, Yb(CF₃ SO₃)₃, Yb(PF₆)₃, Yb(ClO₄)₃,Yb(AsF₆)₃, Yb(SbF₆)₃, La(BF₄)₃, La(CF₃ SO₃)₃, La(PF₆)₃, La(ClO₄)₃,La(AsF₆)₃ and La(SbF₆)₃. The material for the positive electrode isselected from BaSmNiO₅, SmMnO₃, Sm₃ Fe₅ O₁₂, BaEu₂ NiO₅, EuFeO₃, EuFe₅O₁₂, EuMnO₃, EuYbFe₂ O₄, LaNiO₃, La₂ CoO₄, LaNi₀.6 Co₀.4 O₃, LaMnO₄.15,La₄ Mn₄ O₁₁, LaMnO₃, LaMn₇ O₁₂ and LaMnO₃.15.

The above objects are also fulfilled by yet another embodiment of theinvention comprising a secondary battery having an average dischargevoltage of about 4.0 V and comprising a positive electrode having amaterial intercalating alkali earth metal ions reversibly, a negativeelectrode having a carbon composition intercalating alkali earth metalions reversibly and an electrolyte having an organic solvent and asolute. The solute includes an alkali earth metal salt and is composedof ClO₄ ²⁻ ions and cations. The material for the positive electrode isselected from BaFeO₃, SrFeO₃, CaFeO₃ and CaFeO₂.

In the present invention, the carbon composition for the negativeelectrode may be selected from coke, refined, coke with 99% or morepurity, organic compound produced by calcined cellulose, graphite andglassy carbon. Graphite is one of the more suitable materials for thenegative electrode because the amount of intercalating anddeintercalating alkali earth metal ions or lanthanoid metal ions islarger. The graphite preferably has an average granule size ranging from1 μm to 30 μm, a "d" value (d₀₀₂) of a crystal face (002) evaluated byX-ray diffraction ranging from 3.35 to 3.40 and a size of crystallitefor a C-axis direction tested by X-ray diffraction larger than 150 Å.

In the secondary battery, the organic solvent may be selected from agroup consisting of ethylene carbonate (EC), dimethyl carbonate (DMC)and vinylene carbonate (VC). Preferably, the organic solvent may becomposed of a mixture of ethylene carbonate (EC) and dimethyl carbonate(DMC).

In each of the three embodiments described above, i.e. the twoembodiments in which the positive electrode has a material thatintercalates alkali earth metals reversibly and the embodiment in whichthe positive electrode has a material that intercalates lanthanoid metalions reversibly, all of the listed positive electrode materials are themaximum intercalated forms thereof. Compounds in the maximumintercalated form have stable chemical characteristics and are availableon the market in that form.

When the positive electrode material has this form, the battery isdischarged. During charging, the alkali earth metal ions aredeintercalated from the positive electrode and intercalated into thenegative electrode.

Although alkali earth metal ions can be introduced into either apositive electrode or a negative electrode, if introduced into anegative electrode made of a carbon compound, destabilization of thenegative electrode would occur.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other objects, advantages and features of the invention willbecome apparent from the following description thereof taken inconjunction with the accompanying drawings which illustrate specificembodiments of the invention. In the drawings:

FIG. 1 is a cross-sectional view of a secondary battery in accordancewith one exemplary embodiment of the present invention, and

FIG. 2-FIG. 9 are graphs showing the relationship between dischargecapacity and battery voltage of various embodiments of the presentinvention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The following detailed description is believed to be of the bestpresently contemplated mode of carrying out the invention. Thisdescription is not to be taken in a limiting sense, but is made merelyfor the purpose of illustrating general principles of the invention. Thescope of the invention is best defined by the appended claims.

In the present invention, an ion conductive agent is composed of analkali earth metal ion (Ba², Sr²⁺, Ca²⁺, Mg²⁺) or a lanthanoid metal ion(Eu³⁺, Yb³⁺, Sm³⁺, La³⁺ or the like). An ion conductive agent for asecondary battery in accordance with the present invention is used forseveral reasons: Charge of the alkali earth metal ion or the lanthanoidmetal ion is two or three times larger than that of a lithium ion. Thesecations can transfer two to three times more electricity per one ion inthe battery. Therefore, the capacity of the secondary battery usingthose ions for an ion conductive agent will be two to three times largerthan one using lithium.

It is known that alkali earth metal ions are chemically intercalatedinto carbon compound. However, it has not been known that the alkaliearth metal ions are electrochemically intercalated into carboncompounds. This is because an electrolyte having a superior conductivityis required in order to intercalate alkali earth metal ions into carboncompounds. It was thought that no suitable electrolyte was available.

In the present invention, either an alkali earth ion or a lanthanoidmetal ion is limited to an ion conductive agent from the group of 2+charged and 3+ charged metal ions. A reason for the limitation is thatthe oxidation-reduction potential of both the alkali earth ion and thelanthanoid metal ion is baser than that of other 2+ charged or 3+charged metal ions. As a result, a battery with higher capacity will beobtained.

The above positive electrode materials may be mixed with a binder, forexample, polytetrafluoroethylene (PTFE) and polyvinylidenfluoride (PVdF)and a conductive agent, for example, acetylene black, carbon black andgraphite, so as to provide a mixture for the positive electrode.

In one embodiment of the present invention, a carbon composition for anegative electrode which intercalates either alkali earth metal ions orlanthanoid metal ions reversibly may be formed, for example, by coke,preferably, refined coke with the purity of 99% or more, organiccompounds produced by calcined cellulose, graphite or glassy carbon. Itis appreciated that these carbon compositions have a porous structure.In accordance with one aspect of the invention, they may be usedindependently, or two or more of them may be mixed together. Graphitemay be one of the most suitable materials for the negative electrode,because either alkali earth metal ions or lanthanoid metal ions haveboth large intercalating and deintercalating capacities as an activematerial.

More preferably, the graphite would have the following characteristics:

1) an average granule size=1-30 μm,

2) a "d" value (d₀₀₂) of a crystal face (002) evaluated by X-raydiffraction=3.35-3.40,

3) a size of crystallite for a C-axis direction tested by X-raydiffraction=more than 150 Å,

4) a value of specific surface area measured by a BET method=0.5-50 m²/gram, and

5) a value of a real density=1.9-2.3 gram/cm³.

The carbon composition is mixed with a binder, such as for example,polytetrafluoroethylene (PTFE) and polyvinylidenfluoride (PVdF) toobtain a mixture for a negative electrode.

As for a combination of a solute and a positive electrode material, thesolute needs to involve a cation composed of either an alkali earthmetal or a lanthanoid metal for the positive electrode material. Forexample, using BaNiO₃ for the positive electrode material, Ba(ClO₄)₂,Ba(BF₄)₂, Ba(CF₃ SO₃)₂, Ba(PF₆)₂, Ba(AsF₆)₂ or Ba(SbF₆)₂ may be used forthe solute because those are composed of Ba²⁺ cations.

In one aspect of the present invention, an electrolyte includes anorganic solvent, for example, ethylene carbonate (EC), dimethylcarbonate (DMC) and vinylene carbonate (VC). In accordance with oneembodiment of the present invention, a mixture of those solvents may beused. The organic solvent may include vinylene carbonate and itsderivatives. The vinylene carbonate derivatives may be obtained byreplacing at least one of hydrogen atoms of vinylene carbonate by amethyl group, an ethyl group or one halogen atom. It is noted that anorganic solvent used or proposed in a conventional secondary battery foran electrolyte may be applicable. In one embodiment of the presentinvention, a concentration of the solute in the electrolyte is rangingfrom 0.7M to 1.5M, preferably 1M.

EXAMPLE 1 {A Secondary Battery "BA1" of the Present Invention}

A method of preparing a positive electrode will first be described.BaNiO₃ is mixed with graphite powder as a conductive agent in a weightratio of 95:5 to obtain a mixture. The mixture is dispersed in apolyimide resin, linear condensated polyimide macromolecules ("TORAY"TORAYNEECE #3000), with N-methyl-2-pyrrolidone (NMP) as a bindersolution to obtain slurry. The slurry is kneaded and then coated by Dr.blade method onto one side of a conductive plate of thin aluminum foilto obtain an electrode plate. Then, the electrode plate is then dried at60° C. under a vacuum condition to evaporate NMP. On the other side ofthe conductive plate the slurry is coated, and dried in the same way.The polyimide resin is used in 2 parts weight compared to 100 partsweight of the mixture. Last, the plate is heat-treated at a 350° C.temperature for 20 hours to obtain a positive electrode.

A negative electrode may be prepared in the following manner. First,natural graphite powder (a purity of 99% and an average granule size of12 μm) is mixed with a binder solution to obtain a slurry. Second, theslurry is coated onto a current collector comprising a thin copper plateto obtain an electrode plate. Then, the electrode plate is dried andheated to obtain a graphite layer with a 50 μm thickness. In thenegative electrode, the binder is used in 0.5 parts weight in comparisonwith the natural graphite powder of 100 parts weight.

Ethylene carbonate (EC) and dimethyl carbonate (DMC) are mixed to forman organic solvent mixture. An alkali earth metal salt (Ba(ClO₄)₂) isdissolved into the solvent mixture in a concentration of 1 mole/liter toprovide a non-aqueous electrolyte. In the electrolyte, a volume ratio ofEC to DME is preferably set at 1:1.

As shown in FIG. 1, the battery "BA1" of the present invention has apositive electrode 1 and a negative electrode 2. These electrodes 1 and2 are separated by a separator 3 impregnated with the non-aqueouselectrolyte. The separator 3 is typically formed from an ion permeativeporous polypropylene thin film (for example, "Cellgard 3401", made byHoechst Celanese Co. Ltd.). The positive electrode 1, the negativeelectrode 2 and the separator 3 are coiled into a roll, and the roll isplaced in an outer case 7. The positive electrode 1 is connected to apositive terminal 6 through a lead plate 4. The negative electrode 2 isconnected to the outer case 7 through a lead plate 5. The battery "BA1"typically has a cylindrical shape. In a preferred embodiment, thediameter and the height of the battery "BA1" are 14.22 mm and 50 mm,respectively.

EXAMPLES 2-9 {Secondary Batteries "BA2-BA9" of the Present Invention}

All of the elements of batteries "BA2-BA9", except for the positiveelectrode and solute thereof, are the same as those in battery "BA1".The batteries "BA2-BA9" have positive electrodes and solutes as shown inTable 1. In the batteries "BA1-BA9", each battery has a positiveelectrode composed of an alkali earth metal oxide as shown in Table 1.

                  TABLE 1                                                         ______________________________________                                                   Positive            Stopping                                       Battery    electrode  Solute   voltage                                        ______________________________________                                        BA1        BaNiO.sub.3                                                                              Ba(ClO.sub.4).sub.2                                                                    4.2 V                                          BA2        BaCoO.sub.3                                                                              Ba(ClO.sub.4).sub.2                                                                    4.2 V                                          BA3        BaFeO.sub.3                                                                              Ba(ClO.sub.4).sub.2                                                                    4.5 V                                          BA4        SrNiO.sub.3                                                                              Sr(ClO.sub.4).sub.2                                                                    4.2 V                                          BA5        SrCoO.sub.3                                                                              Sr(ClO.sub.4).sub.2                                                                    4.2 V                                          BA6        SrFeO.sub.3                                                                              Sr(ClO.sub.4).sub.2                                                                    4.5 V                                          BA7        CaCo.sub.2 O.sub.4                                                                       Ca(ClO.sub.4).sub.2                                                                    4.5 V                                          BA8        CaFeO.sub.3                                                                              Ca(ClO.sub.4).sub.2                                                                    4.2 V                                          BA9        CaFeO.sub.2                                                                              Ca(ClO.sub.4).sub.2                                                                    4.5 V                                          BC1        LiCoO.sub.2                                                                              LiCl0.sub.4                                                                            4.2 V                                          ______________________________________                                    

EXAMPLES 10-25 {Secondary Batteries "BA10-BA25" of the PresentInvention}

All of the elements of batteries "BA10-BA25", except for the positiveelectrode and solute thereof, are the same as those in battery "BA1".The batteries "BA10-BA25" have positive electrodes and solutes as shownin Table 2. In the batteries "BA10-BA25", each battery has a positiveelectrode composed of an alkali earth metal oxide as shown in Table 2.

                  TABLE 2                                                         ______________________________________                                                 Positive                 Stopping                                    Battery  electrode     Solute     voltage                                     ______________________________________                                        BA10     Ba.sub.2 SmNiO.sub.5                                                                        Sm(ClO.sub.4).sub.2                                                                      3.6 V                                       BA11     SmMnO.sub.3   Sm(ClO.sub.4).sub.2                                                                      3.4 V                                       BA12     Sm.sub.3 Fe.sub.5 O.sub.12                                                                  Sm(ClO.sub.4).sub.2                                                                      3.9 V                                       BA13     EuFeO.sub.3   Eu(ClO.sub.4).sub.2                                                                      3.9 V                                       BA14     BaEu.sub.2 NiO.sub.5                                                                        Eu(ClO.sub.4).sub.2                                                                      3.6 V                                       BA15     Eu.sub.3 Fe.sub.5 O.sub.12                                                                  Eu(ClO.sub.4).sub.2                                                                      3.9 V                                       BA16     EuMnO.sub.3   Eu(ClO.sub.4).sub.2                                                                      3.4 V                                       BA17     Eu.sub.0.5 Yb.sub.0.5 Fe.sub.2 O.sub.4                                                      Yb(ClO.sub.4).sub.2                                                                      3.9 V                                       BA18     LaNiO.sub.3   La(ClO.sub.4).sub.2                                                                      3.6 V                                       BA19     LaNi.sub.0.6 Co.sub.0.4 O.sub.3                                                             La(ClO.sub.4).sub.2                                                                      3.6 V                                       BA20     La.sub.2 CoO.sub.4                                                                          La(ClO.sub.4).sub.2                                                                      3.6 V                                       BA21     LaMnO.sub.4.15                                                                              La(ClO.sub.4).sub.2                                                                      3.4 V                                       BA22     La.sub.4 MnO.sub.11                                                                         La(ClO.sub.4).sub.2                                                                      3.4 V                                       BA23     LaMnO.sub.3   La(ClO.sub.4).sub.2                                                                      3.4 V                                       BA24     LaMn.sub.7 O.sub.12                                                                         La(ClO.sub.4).sub.2                                                                      3.4 V                                       BA25     LaMnO.sub.3.15                                                                              La(ClO.sub.4).sub.2                                                                      3.4 V                                       BC1      LiCoO.sub.2   LiCl0.sub.4                                                                              4.2 V                                       ______________________________________                                    

COMPARATIVE EXAMPLE

All of the elements of battery "BC" used for comparison purpose, exceptfor a positive electrode and a solute thereof, are the same as those inbattery "BA1". The battery "BC" has a positive electrode composed ofLiCoO₂ and a solute composed of LiClO₄.

COMPARISON OF CHARGE/DISCHARGE CHARACTERISTICS

Cycle characteristics of the above described batteries "BA1-BA9","BA10-25" and "BC" were tested. In the tests, the batteries were chargedto a charge stopping voltage shown in Tables 1 and 2 at a chargingcurrent of 500 mA. The batteries were then discharged to a dischargestopping voltage of 2.0 V at a discharge current of 500 mA to complete 1cycle. FIG. 2-FIG. 9 show discharge characteristics of the batteriestested. In FIG. 2-FIG. 9, the horizontal axis represents batterycapacity (mAh) and the vertical axis represents battery dischargevoltage (V). In Table 3 and Table 4, average discharge voltages (V) andcapacities (mAh) of the batteries are shown.

                  TABLE 3                                                         ______________________________________                                                   Average discharge                                                                          Discharge                                             Battery    voltage (V)  capacity(mah)                                         ______________________________________                                        BA1        3.7          1200                                                  BA2        3.7          1210                                                  BA3        4.0          1150                                                  BA4        3.7          1240                                                  BA5        3.7          1100                                                  BA6        4.0          1120                                                  BA7        3.7          1230                                                  BA8        4.0          1270                                                  BA9        4.0          1100                                                  BC         3.7           600                                                  ______________________________________                                    

                  TABLE 4                                                         ______________________________________                                                   Average discharge                                                                          Discharge                                             Battery    voltage (V)  capacity(mAh)                                         ______________________________________                                        BA10       3.1          1800                                                  BA11       2.9          1810                                                  BA12       3.4          1850                                                  BA13       3.4          1840                                                  BA14       3.1          1700                                                  BA15       3.4          1720                                                  BA16       2.9          1830                                                  BA17       3.4          1870                                                  BA18       3.1          1700                                                  BA19       3.1          1880                                                  BA20       3.1          1815                                                  BA21       2.9          1805                                                  BA22       2.9          1810                                                  BA23       2.9          1820                                                  BA24       2.9          1835                                                  BA25       2.9          1830                                                  BC         3.7           600                                                  ______________________________________                                    

According to FIG. 2-FIG. 4 and Table 3, the batteries "BA1-BA9" usingalkali earth metal ions for an ion conductive material haveapproximately twice the capacity of the battery "BC" used forcomparison.

Furthermore, according to FIG. 5-FIG. 9 and Table 4, the batteries"BA10-BA25" using lanthanoid metal ions for an ion conductive materialhave approximately three times the capacity of the battery "BC".

The "stopping voltage" mentioned herein is the voltage at which thebattery is fully charged, i.e. continued application of charging currentdoes not result in any additional charge storage. The value of thestopping voltage of each battery is determined on the basis ofprinciples known in the art.

A battery needs to be fully or essentially charged in order to dischargethe ions from a positive electrode material. As the ions are absorbed orintercalated into a negative electrode material, the stopping voltage ofthe battery depends upon the negative electrode material. Furthermore,the stopping voltage conditions are also dependent upon the combinationof positive electrode material, negative electrode material and thesolute of ions.

All battery charging/discharging tests described herein were performedat the same temperature, which is necessary to allow valid comparisons.The specific temperature employed to obtain the results described hereinwas normal room temperature.

All of the batteries described herein, BA1-BA25 and BC, contained thesame amount of active material and the same cell volume, as required topermit valid comparisons. Specifically, each of those batteries contains7 grams of active material.

In the above described embodiments, the batteries have a cylindricalshape. However, it should be appreciated that a coin shaped battery or aflat shaped battery are similarly available.

While the description above refers to particular embodiments of thepresent invention, it will be understood that many modifications may bemade without departing from the spirit thereof. The accompanying claimsare intended to cover such modifications as would fall within the truescope and spirit of the present invention.

The presently disclosed embodiments are therefore to be considered inall respects as illustrative and not restrictive, the scope of theinvention being indicated by the appended claims, rather than theforegoing description, and all changes which come within the meaning andrange of equivalency of the claims are therefore intended to be embracedtherein.

What is claimed is:
 1. A secondary battery comprising:a positiveelectrode having a material containing reversibly intercalatedlanthanoid metal ions, said material being selected from the groupconsisting of BaSmNiO₅, SmMnO₃, Sm₃ Fe₅ O₁₂, BaEu₂ NiO₅, EuFeO₃, EuFe₅O₁₂, EuMnO₃, EuYbFe₂ O₄, LaNiO₃, La₂ CoO₄, LaNi₀.6 Co₀.4 O₃, LaMnO₄.15,La₄ Mn₄ O₁₁, LaMnO₃, LaMn₇ O₁₂ and LaMnO₃.15 ; a negative electrodehaving a carbon composition intercalating lanthanoid ions reversibly;and an electrolyte having an organic solvent and a solute, said soluteincluding a lanthanoid metal salt.
 2. A secondary battery according toclaim 1, wherein said solute is selected from the group consisting ofSm(BF₄)₃, Sm(CF₃ SO₃)₃, Sm(PF₆)₃, Sm(ClO₄)₃, Sm(AsF₆)₃, Sm(SbF₆)₃,Eu(BF₄)₃, Eu(CF₃ SO₃) , Eu(PF₆)₃, Eu(ClO₄)₃, Eu(AsF₆)₃, Eu(SbF₆)₃,Yb(BF₄)₃, Yb(CF₃ SO₃)₃, Yb(PF₆)₃, Yb(ClO₄)₃, Yb(AsF₆)₃, Yb(SbF₆)₃,La(BF₄)₃, La(CF₃ SO₃)₃, La(PF₆)₃, La(ClO₄)₃, La(AsF₆)₃ and La(SbF₆)₃. 3.A secondary battery according to claim 1, wherein said organic solventis selected from the group consisting of ethylene carbonate (EC),dimethyl carbonate (DMC) and vinylene carbonate (VC).
 4. A secondarybattery according to claim 1, wherein said carbon composition isselected from the group consisting of coke, refined coke with ₉₉ % ormore purity, organic compound produced from calcined cellulose, graphiteand glassy carbon.